1. Field of the Invention
The present invention relates to a display and its driving method and, more particularly, to a display for inputting an image signal of an AC voltage to each pixel and its driving method.
2. Related Background Art
In recent years, multimedia has become increasingly important and the amount of information that is handled in the society is rapidly increasing. In such a situation, in place of a CRT (Cathode Ray Tube), a thin type flat display as an interface from a computer to a human being has become an important device to widen the multimedia market. As flat displays, LCD (liquid crystal display), PDP (plasma display), and an electron beam flat displays are leading devices. Among them, the liquid crystal display is achieving a wider market in association with a widespread use of small personal computers. Among the liquid crystal displays, the active matrix liquid crystal display has no crosstalk as compared with a simple matrix liquid crystal display of an STN (super twisted nematic) type or the like, so that the active matrix LCD has a large contrast over the whole picture plane. Such an active matrix LCD is, therefore, has attracted use as not only a display of the small type personal computer but also for use as a view finder of a video camera, a projector, and a thin type television.
Among active matrix liquid crystal displays, there are TFT (thin film transistor) type displays and diode type displays. FIG. 10A is a block diagram of an image signal input of a TFT type image display. Reference numeral 10 denotes an image pixel section having pixels arranged in a matrix shape; 20 a vertical scanning circuit for selecting a display row; 30 a sampling circuit of a color image signal; and 40 a horizontal scanning circuit for generating a signal of the sampling circuit.
A unit pixel of the display pixel section 10 comprises a switching element 11, a liquid crystal material 15, and a pixel capacitor 12. In the case where the switching element 11 is a TFT (thin film transistor), a gate line 13 connects a gate electrode of the TFT and the vertical scanning circuit 20. A common electrode 21 of an opposite substrate commonly connects terminals of one side of the pixel capacitor 12 of all of the pixels. A common electrode voltage V.sub.LC is applied to the common electrode 21. When the switching element 11 is a diode (including a metal/insulator/metal element), a scan electrode is arranged in the lateral direction on the opposite substrate and is connected to the vertical scanning circuit 20. An input terminal of the switching element 11 is connected to the sampling circuit 30 by a data line 14 in the vertical direction. In the case where the switching element 11 is any one of the TFT and the diode, the vertical direction data line 14 connects the input terminal of the switching element 11 and the sampling circuit 30. An output terminal of the switching element 11 is connected to another terminal of the pixel capacitor 12.
A control circuit 60 separates an image signal to signals necessary to the vertical scanning circuit 20, horizontal scanning circuit 40, a signal processing circuit 50, and the like. The signal processing circuit 50 executes a gamma process considering liquid crystal characteristics, an inverting signal process to realize long life of the liquid crystal, and the like and generates color image signals (red, blue, and green) to the sampling circuit 30.
FIG. 10B is a detailed equivalent circuit diagram of the color display pixel section 10 of the TFT type and the sampling circuit 30. The pixels (R, G, B) are arranged in a delta shape and the pixels of the same color are distributed to both sides of the data lines 14 (d1, d2, . . . ) every row and are connected to the data lines (d1, d2, . . . ). The sampling circuit 30 is constructed by switching transistors (sw1, sw2, . . . ) and a capacitor (a parasitic capacitance of the data lines 14 and a pixel capacitance). An image signal input line 16 is constructed by signal lines only for R, G, B colors. The switching transistors (sw1, sw2, . . . ) sample the color signals of the image signal input line 16 in accordance with pulses (h1, h2, . . . ) from the horizontal scanning circuit 40 and transfer the color signals to the pixels through the data lines 14 (d1, d2, . . . ). Pulses (.phi.g1, .phi.g2, . . . ) are transmitted from the vertical scanning circuit 20 to TFT gates of the pixels and rows are selected, thereby writing the signals to the pixels. As mentioned above, the pulses (.phi.g1, .phi.g2, . . . ) turn on the TFTs 11 included in the rows, so that an image signal of one horizontal scan of each corresponding row is written to all of the pixels included in the rows. The image signal of one horizontal scan is called a 1H signal hereinbelow.
FIG. 11A shows an example of an interlace scan of a liquid crystal display having rows of the same number as that of the vertical scanning lines of an image signal for a CRT type television based on the NTSC or the like. In the liquid crystal display, when the 1H signal is written to two rows, to decrease flickering of a moving image, 2-row simultaneous driving or a 2-row interpolation driving (signal writing corresponding to the pixels arranged in a delta shape) which is treated similarly to the 2-row simultaneous driving, is often executed. In those driving methods, a combination of two rows to be selected is changed in accordance with the odd field and the even field. In the following description, it is assumed that the rows on the display pixel section which are selected and to which information is written are designated by symbols (g1, g2, . . . ) of vertical scanning pulses. In the odd field, the 1H signal of a horizontal scan line odd1 is written to the rows g2 and g3. Similarly, the 1H signal of odd2 is written to the rows g4 and g5. Each of the 1H signals of odd3 and subsequent horizontal scan lines is also similarly written for every two rows. On the other hand, in the even field, a combination of rows to be selected is deviated from the odd field by one row and the 1H signal of a horizontal scan line even1 is written to the rows g1 and g2. Similarly, the 1H signal of even2 is written to the rows g3 and g4 and each of the subsequent signals is also similarly written for every two rows.
FIG. 12 shows a timing chart of scan pulses of the 2-row simultaneous driving. In the odd field, the vertical scan pulses .phi.g2 and .phi.g3 are set to the "H" level. The TFT corresponding to each of the pixels of the rows is turned on, thereby writing the 1H signal of odd1 to the rows g2 and g3. In this instance, for the "H" period of the horizontal scan pulses (h1, h2, . . . ), the image signal sampled by the sampling circuit is written to the pixels of the rows g2 and g3. A similar writing operation is also executed in the scan of odd2 and subsequent lines.
FIG. 11B shows an example of the interlace scan of a liquid crystal display having rows of the number that is 1/2 of the number of vertical scan lines of the image signal for the CRT type television based on the NTSC or the like. In this case, the rows to be selected on the display pixel section are also shown by the symbols (g1, g3, . . . ) of the horizontal scan pulses. In the odd and even fields, the 1H signal is written to the same row. In the odd field, the 1H signal of the horizontal scan line odd1 is written to the row g2 and the 1H signal of odd2 is written to the row g4. Similarly, each of the 1H signals of odd3 and subsequent lines is also written. In the even field as well, the 1H signal of even1 is written to the row g2 and the 1H signal of even2 is written to the row g4. Each of the subsequent signals is also similarly written by using rows (g4, g8, . . . ) to which the information was written in the odd field. A timing chart of the scan pulse shows a scan by the 2-row simultaneous driving shown in FIG. 12 without the odd row pulses (.phi.g7 g3, .phi.g5, . . . ).
In the liquid crystal display, when a predetermined voltage is applied to a liquid crystal material for a long time, a burning phenomenon may occur such that quality of the liquid crystal material is diminished. Therefore, the image signal is written from the reference potential by the positive or negative polarity, thereby executing an AC driving in which the polarities of the image signal are exchanged. When an exchanging period of the signal polarities is long, a flickering that is visibly recognized by the eyes of a human being appears. FIG. 13A shows signal polarities of selected rows in the 2-row simultaneous driving. A case where the voltage of the image signal is positive for the common electrode voltage as a reference potential is expressed by "+" and a case where it is negative is expressed by "-". Each field scan period is shown in the lateral direction. A selected row is shown in the vertical direction. The signal polarities are exchanged every horizontal scan. In this case, when attention is paid to one selected row (for example, row g2), the signal polarities are inverted every two fields. Therefore, a line flicker of 30 Hz of 1/2 of the scan period (60 Hz) of one field occurs and becomes a flickering of the display. As the frequency of the flicker is low, the flicker is recognized to the human eyes and becomes conspicuous. Particularly, when the flicker period decreases to 50 Hz or less, it is seen as a flicker to the human eyes. Therefore, there is an example such that the signal polarity of each row is inverted every field and the flicker period is set to 60 HZ. FIG. 13B shows the 2-row simultaneous driving in which the signals of the same polarity are written in the odd fields and the signals of different polarities are written in the even fields and the signal polarities are exchanged every field when an attention is paid to any row. In this case, the flicker period is set to 60 Hz and is hard to be recognized to human eyes.
In the AC driving, the flicker is made inconspicuous by reducing the writing period of the signal to the pixel. However, a case exists where even if the writing period is set to the shortest period, when still information such as a character or the like is displayed for a long time, burning of the liquid crystal material occurs. For example, the case where the whole picture plane is displayed in black by the 2-row simultaneous driving and only a certain portion is displayed in white will now be considered. First, attention is paid to an example of the scan when an NTSC signal is displayed at a high fidelity to a CRT television or a display that is almost equivalent thereto. FIG. 14 shows an example of such a scan. In FIG. 14, scan lines even2, odd2, and even3 denote 1H signals of the white display and the other scan lines indicate black display signals (the signals of the black display are omitted). Since those displays display the original image signal as it is at a high fidelity, by performing AC driving, even if a still image is displayed, there is no fear of occurrence of the burning of the liquid crystal material.
FIG. 15A shows an example of a scan when the same NTSC signal is displayed by the 2-row simultaneous driving. In the odd field, the 1H signal (original signal o2, pseudo signal o'2) of odd2 is written to the rows g4 and g5. In the even field, the 1H signal (original signal eZ, pseudo signal e'2) of even2 is written to the rows g3 and g4. The 1H signal (original signal e3, pseudo signal e'3) of even3 is written to the rows g5 and g6. In this instance, the signal which is inverted every field is written to each row. FIG. 15B shows a signal voltage waveform of each row. The upper side than the reference potential (V.sub.LC) shows an odd field period of FIG. 15A. The lower side shows an even field period. The rows in which the white display signal was written in the odd field period are only the rows g4 and g5. The rows in which the white display signal was written in the even field period are the four rows g3, g4, g5, and g6. In this instance, the rows g3 and g6 are displayed in black in the odd field and are displayed in white in the even field. Namely, the voltages of the hatched portions remain as DC voltages in the liquid crystal. When such a state is left for a long time, even if AC driving is executed, there is a fear of occurrence of burning of the liquid crystal material.
FIG. 16A shows an example of a scan when the NTSC signal is displayed by a liquid crystal display in which the number of rows is only 1/2 of the number of scan lines of the signal as described in FIG. 5. The 1H signal of odd1 and the 1H signal of even1 are written to the same row g2 and the signals of odd2 and even2 are written to the same row g4. The signals are subsequently written in a manner similar to the above. even2, odd2, and evens show white display signals and the other scan lines show black display signals. FIG. 16B shows a signal voltage waveform of each row. In this case as well, in the row g6, the voltage of the hatched portion remains as a DC voltage in the liquid crystal and if such a state is left for a long time, there is a fear of occurrence of burning of the liquid crystal material. Even in the plasma display, electron beam flat display, and electroluminescence display, there is a case where the devices are deteriorated such that the electrodes are corroded or the like in DC driving, and there is a case where the AC driving is performed. Consequently, in a manner similar to the liquid crystal display as described above, when a still image is inputted, even if the AC driving is executed, the DC voltage remains and there is a fear of deterioration of the device.
To solve the above problems, there is a liquid crystal display such that a television signal which handles a motion image is 2-line simultaneous interlace driven and a still image such as character information or the like is 2-line simultaneous non-interlace driven (Japanese Laid-Open Patent Application No. 3-94589). However, in such a liquid crystal display, if there is a still image portion in the television signal, a burning occurs. To prevent it, it is necessary to use a frame memory, a motion detecting circuit, or the like to judge whether the image is a motion image or a still image, so that the apparatus becomes very complicated and expensive.